1. Field of the invention
The present invention relates to an improvement to electron tubes, more especially to power electron tubes operating at frequencies of the order of a few hundred megahertz.
The present invention relates more particularly to a means for internally cooling the base of some electrodes of the tube.
2. Description of the prior art
As shown schematically in FIG. 1 which relates to a power tetrode, the electron tubes to which the present invention applies are vacuum tubes formed essentially by coaxial cylindrical electrodes comprising an anode 1, a screen grid 2 called grid G.sub.2, a control grid 3 called grid G.sub.1 and a cathode 4.
These different electrodes are connected to the outside of the tube through circular metal connections 5, 6, 7, 8 separated from each other by insulators 9, 10, 11, 12 preferably made from a ceramic material and further providing sealing of the tube. These metal connections 5, 6, 7, 8 are generally formed by pieces stamped in the shape of cups and are brazed to the insulators.
The metal connections are connected to different voltage sources not shown and serve respectively for the passage of the current for heating the cathode and the circulation of high frequency currents.
However, heating of the cathode and circulation of the high frequency currents are heat generators and this heat is removed by conduction towards the metal connections.
Usually, the connections are cooled by injecting compressed air on the head of the tube. In most cases, this cooling is sufficient for maintaining the connections and the brazing of these connections to the insulators at a sufficiently low temperature which does not damage them.
However, the ultra high frequency operation of this type of tube gives rise to a sinusoidal distribution of the surface electric currents. Consequently, some zones of these surfaces which correspond to a current "antinode" where the intensity is maximum are subjected to intense local heating.
In some cases of operation, these current antinodes are situated at the level of the connections. Consequently, French patent application No. 81 21804 has proposed a cooling system outside the tube formed by a spiral shaped tube through which a cooling fluid flows and in engagement, preferably by welding, with the connection of the electrode to be cooled.
With this cooling system, a considerable amount of heat is eliminated, in particular in the vicinity of a current "antinode".
However, the heating zones due to current "antinodes" located inside the tube are not cooled. Now, in some cases, the heating is such that it brings the metal parts up to a high temperature, resistance to the passage of the current increasing the temperature. Thus, the increase in temperature may be such that the vapor tensions of the metals from which the electrodes are formed are reached. In this case, there is an emission of gas which causes at least a local deterioration of the vacuum and renders the tube unserviceable.
Furthermore, in power electron tubes of known type, the hear generated at the grids, and more particularly at the control grid G.sub.1, by the current "antinodes" can only be removed by radiation from the envelope of the tube or by conduction through the output connection. Now, the connections are generally made from an iron-nickel-cobalt alloy of small thickness for readily brazing them to the contiguous ceramic material insulators. Now, iron-nickel-cobalt alloy is a poor heat conductor. Moreover, the connections have small thicknesses. The result is therefore very limited removal of the heat.